I think you've got suspension of disbelief backwards. It's more about the tendency or willingness of a reader/viewer/user to believe the story
in spite of the implausibility of the details. In fact, I just checked and that's almost exactly how
wikipedia words it.
The problem with these discussions is that they hinge more on the (often incorrect) assumptions of the reader/viewer/user than they do on "reality". Take your comments, for example:
1. The assault rifle doesn't look anything like the M16 or the AK47, the two most prominent assault rifles in use in the 1960s. It actually bears greater similarities in appearance to the modern-era
G36. Neither does the sniper rifle look like the M14 or Dragunov used at that time. It looks more like the
Barrett M82 with a muzzle break, also in contemporary usage.
2. If you haven't noticed, Phalanx bases are underground. Aircraft must be capable of VTOL (vertical take off and landing).
3. Why do you assume aliens care more about Phalanx than killing civilians? You could call it unrealistic, but a sci-fi author could have just as easily intended it to be an insight into alien psychology and strategy.
You can pick any single element in the game (or any sci-fi work) and find a reason why it might not make sense. The same is true in the world we live in. Suspension of disbelief is about embracing the whole rather than picking on the details. If we have an engrossing game, we'll succeed in generating that.
But there will always be people who pick out one thing and decide
that is the reason the game is unrealistic. Everyone finds something different for their brain to fixate on. Changing one thing or even a list of things won't fix the problem. It will just introduce new ones.